The present invention relates to a liquid crystal display apparatus of active matrix type and a driving method therefor.
FIG. 13 is a diagram of a driving circuit of a conventional active matrix type liquid crystal display apparatus as described in Japanese Unexamined Patent Publication No. 313607/1993. In this diagram, a plurality of X electrode lines (Xi−1, Xi, Xi+1 . . . ) and Y electrode lines (Yj−1, Yj, Yj+1 . . . ) are arranged in a form of a matrix, and active elements 11 and liquid crystal elements 12 such as TFT (thin film transistors) are formed on intersections of each of the X electrode lines and Y electrode lines. The Y electrode lines are also called data lines and are connected to a display signal circuit 13 for outputting display data signals for each of the liquid crystal display elements 12. Further, the X electrode lines are also called scanning signal lines and are connected to a scanning signal circuit 14 for outputting scanning signals.
The counter side of the liquid crystal display elements 12 are connected to a common electrode 15. The driving of the active elements 11 is performed in that the active elements 11 on the X electrode lines are set to at ON conditions (active conditions) synchronously with the scanning of the X electrode lines, in that display data signals are output from the display signal circuit 13, and in that data signals are written into corresponding liquid crystal display elements 12 via the active elements 11 in ON conditions. It should be noted that there have also been taken measures in which storage capacitances 16 are provided, upon requirement, for the liquid crystal display elements 12 in order to improve storage characteristics of electrical charge of the liquid crystal display elements 12.
An example of a conventional driving method for an active matrix type liquid crystal display apparatus is disclosed in Japanese Unexamined Patent Publication No. 141269/1994, and FIG. 14 is a view showing an example of a timing chart for indicating the driving method. As known, liquid crystal need to be driven through alternating-current, whereby an electric potential 111 of the signal line is made to be an image signal performing alternating-current inversion with a certain electric potential Vc being the center. During vertical scanning period T1, an electric potential 112 of a scanning line becomes high-leveled by a single scanning period T3. Such a scanning pulse is sequentially applied from above the screen per scanning line. T2 denotes a vertical blanking period (hereinafter also referred to as mere “blanking period”) in which usually no image signals are applied. An electric potential of a counter electrode 113 is set to be lower than the central electric potential Vc of the image signal in case of N channel TFTs.
It will now be explained for a line common inversion driving method that is one of the objects of the present invention as a driving method for the above liquid crystal display apparatus. In a line common inversion driving method, two adjacent pixels are driven through alternating-current to be of opposite polarity, and this method is advantaged in that a driving IC of low cost may be employed and the power consumption can be decreased.
Driving waveforms of a conventional TFT-LCD employing the line common inversion method is shown in FIG. 15 and FIG. 16. FIG. 15 is a view of driving waveforms of odd-numbered lines and FIG. 16 of driving waveforms of even-numbered lines, respectively. In FIGS. 15 and 16, Vd denotes electric potentials of drain electrodes (broken line of short pitches), Vcom electric potentials of the counter electrodes (thin real line), Veff voltage applied onto the liquid crystal (potential differences between Vd and Vcom are shown by the hatching), Vg electric potentials of the gate lines including voltage at the time of gate OFF Vgl and voltage at the time of gate ON Vgh. Vs denotes electric potentials of source lines (broken line of long pitches). In case the reference marks Vcom, Vg, Vs are indicated in connection with the word “signal” such as “Vcom signal”, these represent signals having electric potentials of counter electrodes. Further, DA denotes data period, and BK blanking period, respectively. The effective voltage Veff that is applied on the liquid crystal corresponds to a root-mean-square of a single frame period of an electric potential difference between Vd and Vcom. The Vd varies per Single Horizontal period (1H) depending on the Vcom, Vg and Vs signals.
In alternating-current driving based on a line common inversion method, Vg is controlled by the scanning signal circuit, Vs by the display signal circuit, Vcom by a timing control circuit and a power source circuit (not shown), while Vd is determined by the Vg, Vs and Vcom. A Single Horizontal period (1H) is approximately 32 μs in case of VGA, approximately 26 μs in case of SVGA, and 20 μs in case of XGA, and a value for each of the electric potentials Vgh is set to be a voltage with which charge/discharge of electric charge of the drain electrodes can be completed within 1H, that for Vgl to be a voltage with which the electric charge of the drain electrodes can be sufficiently held during a single frame period, and those for Vs and Vcom to be a voltage with which display can be performed at a desired luminance.
Since this variation is repeated per 1H during the data display period, the Veff of the odd-numbered lines and even-numbered lines can be set to be identical by optimizing the central value for the Vcom. However, since the Vcom, Vgl and Vs signal are usually not varied but remain fixed during the blanking period, the drain variation at the start of the blanking period is maintained during the blanking period whereby luminance differences are generated line by line owing to the different values for the Veff of the odd-numbered lines and even-numbered lines during the blanking period as shown in FIG. 15 and FIG. 16. The relationship between Vd and Vcom will now be explained. As shown in FIG. 17, the electric potential Vd of the pixel electrodes (drain electrodes) changes owing to effects of variations in (1) electric potential Vcom of the counter electrodes 22, (2) electric potential of a storage electrodes 23, and (3) electric potential Vs of the source electrodes in a holding condition of the TFTs 21. However, the electric potential of the storage electrode is determined by the Vg, Vcom and other factors, and a signal identical in amplitude and polarity with those of the Vcom (while the DC values may be different) is applied. In this manner, since the alternation of (1) to (3) is terminated during the blanking period, it may happen that the luminance differences of brightness between odd-numbered lines and even-numbered lines are generated.